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Ian, to answer your questions.. 1) NO. Unless you alter the physical dimentions, or resort to some very exotic ( unobtanium ?) alloys, you cannot change the stiffness by more than 10% across the whole range of steel alloys and heat treatments.
Remember , this is all assuming we are talking in the classical accepted engineering terminology which is based on static analysis. Once things start being "dynamic" ( rotating axles) then other factors come into play, and this is why i said there is a confusion of terms. some people refer to axle "stiffness", others refer to "hardness" grades ( others simple refer to a colour code ?)
2) I dont know if a 10% change would be significant enough to detect on the track Now, imagine a typical axle on the bench with 3 bearing cassettes fitted..one each end and one in the center. Fix one end and the center to the bench with the other end overhanging the bench. Hang a 50kg weight from the free end cassette and measure accurately the deflection. Lets assume it deflects 1.0mm. now rotate the axle slowly and note the deflection as it rotates. If the axle is well made there will be no change in the deflection . BUT, what happens if you rotate the axle at 1000 or 2000, or 4000 rpm ?? ...there will be a reduction in the deflection ? This is a DYNAMIC effect on the apparent stiffness and is proportional to speed and a function of several material properties... such as damping effects and vibrational properties. Now, it is known that HARDNESS ( heat treatment) will change these vibration characteristics, so it is reasonable to expect that "hardness" could well alter the dynamic behaviour of the axle significantly. ..I dont know, i have never tested axles to that extent in reality.
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